274 HUMAN PHYSIOLOGY. 



The lungs are developed from the anterior part of the esophagus. 

 At first a small bud appears, which, as it lengthens, divides into two 

 branches ; second and tertiary processes are given off from these, 

 which form the bronchial tubes and air-cells. The lungs originally ex- 

 tended into the abdominal cavity, but became confined to the thorax 

 by the development of the diaphragm. 



The bladder is formed by a dilatation of that portion of the 

 allantois remaining within the abdominal cavity. It is at first pear- 

 shaped and communicates with the intestine, but later becomes sep- 

 arated and opens exteriorly by the urethra. It is attached to the 

 abdominal walls by a rounded cord the urachus, the remains of a 

 portion of the allantois. 



Genito-urinary Apparatus. The Wolffian bodies appear about the 

 thirteenth day, as long, hollow tubes running along each side of the 

 primitive vertebral column. They are temporary structures, and are 

 sometimes called the primordial kidneys. The Wolffian bodies con- 

 sist of tubules which run transversely and are lined with epithelium ; 

 internally they become invaginated to receive tufts of blood-vessels ; 

 externally they open into a common excretory duct, the duct of the 

 Wolffian body, which unites with the duct of the opposite body and 

 empties into the intestinal canal at a point opposite the allantois. 

 On the outer side of the Wolffian body there appears another duct, 

 the duct of Miiller, which also opens into the intestine. 



Behind the Wolffian bodies are developed the structures which 

 become either the ovaries or testicles. In the development of the 

 female the Wolffian bodies and their ducts disappear ; the ex- 

 tremities of the Miillerian ducts dilate and form the fimbriated 

 extremity of the Fallopian tubes, while the lower portions coalesce 

 to form the body of the uterus and vagina, which now separate them- 

 selves from the intestine. 



In the development of the male the Miillerian ducts atrophy, and 

 the ducts of the Wolffian body ultimately form the epididymis and 

 vas deferens. About the seventh month the testicles begin to descend, 

 and by the ninth month have passed through the abdominal ring into 

 the scrotum. 



The kidneys are developed out of the Wolffian bodies. They con- 

 sist of little pyramidal lobules, composed of tubules which open at 

 the apex into the pelvis. As they pass outward they become con- 

 voluted and cup-shaped at their extremities, receive a tuft of blood- 

 vessels, and form the Malpighian bodies, 



